The present invention relates generally to ophthalmic non-contact tonometers of a type operable to discharge an air pulse at an eye to applanate an area of the cornea for measuring intraocular pressure (IOP), and relates more specifically to a calibration apparatus and method for non-contact tonometers of the stated type.
Existing non-contact tonometers measure IOP by activating a pump mechanism to fire an air pulse at the cornea to flatten or xe2x80x9capplanatexe2x80x9d a predetermined area of the cornea, detecting corneal applanation caused by the air pulse and a plenum pressure of the pump mechanism, and correlating the plenum pressure at the moment of applanation with IOP. In older instruments, the time elapsed to achieve applanation was correlated to IOP as an xe2x80x9cindirectxe2x80x9d representation of plenum pressure based on a linearly increasing pressure profile in the plenum. In present day instruments, a pressure sensor is mounted in the plenum for providing a signal proportional to the plenum pressure. Regardless of whether elapsed time or a signal from a pressure sensor is obtained, it is necessary to correlate the obtained quantity to IOP such that the instrument provides a meaningful measurement value of IOP as output. Thus, non-contact tonometers must be calibrated periodically to ensure that the correlation function used by the particular instrument yields IOP results that are substantially in agreement with an established standard of IOP measurement.
Traditionally, the Goldmann applanation tonometer(GAT), which measures IOP by directly contacting the cornea to applanate an area of the cornea, has been used as a standard for IOP measurement. Accordingly, initial calibration of a non-contact tonometer has been carried out by way of a clinical trial involving a large number of human eyes. During the clinical trial, each eye is measured with both GAT and the subject non-contact tonometer, and the parameters of a correlation function of the subject non-contact tonometer are adjusted to provide a best fit to the GAT results.
Conducting clinical trials is time consuming and expensive, and therefore clinical calibration might be conducted with respect to a xe2x80x9cmasterxe2x80x9d non-contact tonometer, and the master non-contact tonometer is then used as a calibration standard. It is known to measure xe2x80x9cIOPxe2x80x9d of a set of precision-manufactured rubber eyes designed and tested to applanate at predetermined pressures as a calibration gauge to avoid a clinical trial involving human eyes. Rubber eyes develop folds during testing and tend to be a poor simulation of a real eye. Moreover, rubber eyes are expensive and difficult to manufacture because very tight tolerances are necessary. Finally, the rubber material ages or can be damaged, so that a set of rubber eyes must be constantly recalibrated to maintain reliability.
The Physikalisch-Technische Bundesanstalt (PTB) of Germany has developed a calibration tool for non-contact tonometers that employs a mirror and lever system, wherein the tonometer air pulse is directed at a mirror mounted on a lever to angularly displace the lever about a pivot axis. A working version of the tool incorporates a complex assembly of precision moving parts and is available at a cost of close to $30,000.00.
Finally, Japanese Patent No. 11-225974 describes another mechanical calibration tool generally similar in concept to the PTB calibration tool in that it comprises a mirror mounted for measurable deflection by a tonometer air pulse.
Thus, tonometer calibration devices and methods of the prior art are delicate, expensive, unstable, and/or difficult to use, and they cannot be traced to an absolute pressure standard such as that provided by a water column or precision pressure calibrator.
Therefore, it is an object of the present invention to provide an apparatus for calibrating a non-contact tonometer that is inexpensive and reliable over time.
It is another object of the present invention to provide an apparatus for calibrating a non-contact tonometer that is easy to operate.
It is another object of the present invention to provide an apparatus for calibrating a non-contact tonometer that is traceable to an absolute pressure standard.
It is a further object of the present invention to provide a tool and method for calibrating a non-contact tonometer that minimizes influence of an operator""s skill level.
An apparatus of the present invention for calibrating a non-contact tonometer comprises a calibration tool mountable in front of an air tube of a non-contact tonometer. The calibration tool includes xe2x80x9can electronic eyexe2x80x9d having a pressure sensor for receiving the air pulse and applanation simulation means connected to the pressure sensor for providing a pseudo-applanation event, such as an infra-red pulse, when the pressure sensor signal reaches a predetermined level. The pressure sensor includes a piezoresistive semiconductor sensing element covered by a polymer gel for reducing flow noise in the sensor signal. In addition to the electronic eye, the calibration tool preferably includes a glass sphere for use in aligning the non-contact tonometer, a planar mirror for use in setting proper angular orientation of the calibration tool about X and Y axes of the system, and a rubber eye for use in conducting a known xe2x80x9cfire aroundxe2x80x9d offset test. The electronic eye, glass eye, mirror, and rubber eye are carried on a precision slide mechanism making it easy to manually slide a new station into position in front of the tonometer air tube.
The calibration apparatus further comprises a controller unit in a housing remote from the calibration tool mounted in front of the tonometer. The controller enables adjustment of the threshold pressure signal voltage between settings corresponding to low, medium, and high IOP calibration values. The controller also allows for input of a signal from an external calibration device.
The invention also encompasses a method of calibrating a non-contact tonometer using the tonometer calibration apparatus. The method preferably comprises the steps of operating the non-contact tonometer to direct an air pulse onto a pressure sensor; comparing a pressure signal from the pressure sensor with a predetermined signal level corresponding to a known IOP measurement standard; inducing the non-contact tonometer to detect applanation when the pressure signal from the pressure sensor reaches the predetermined signal level such that the non-contact tonometer provides a measured pressure value; repeating the aforementioned steps for other known IOP measurement standards; and adjusting at least one parameter of a correlation function of the non-contact tonometer to reduce a difference between the measured pressure values and the known IOP measurement standards.